1. Field of the Invention
This invention relates to an apparatus for precisely transferring a negative to a photosensitive member, and more particularly relates to an apparatus which effects the transfer by scanning a wafer with a semiconductor integrated circuit pattern image.
2. Description of the Prior Art
In the field of semiconductor elements, there is a tendency to more minute patterns and higher integration, such as IC, LSI and super-LSI, and therefore higher accuracy and higher quality have been required of printing apparatus.
Important factors required of the printing apparatus will hereinafter be described.
(a) Printing performance for enabling minute patterns of the order of 1-2 m to be printed.
(b) Positioning accuracy necessary for accurately positioning a pattern of a photomask of the next step relative to the pattern on a wafer which has been printed by the photomask of the preceding step.
As an apparatus which satisfies the above-mentioned requirements, there is known one in which the slit image of a photomask is projected upon a wafer by means of a reflecting optical system, comprising combined concave and convex mirrors, and in which the photomask and the wafer are moved together with each other for the whole surface exposure of the wafer. In this apparatus, it is important to move the photomask and wafer smoothly, and a construction for realizing this is described in U.S. Pat. No. 4,215,934. Also, an air bearing system for placing a movable portion thereon to ensure highly accurate movement is described in U.S. Pat. No. 4,215,934.
In FIG. 10 of the accompanying drawings, reference numeral 31 designates a mask illuminating system which comprises, in the direction of the optical axis, a condensing mirror 33, a mercury lamp 34, a first condenser lens 35, a filter 32, a mirror 36, a second condenser lens 37 and a light-intercepting plate 38 having an arcuate slit. The filter 32 is inserted during alignment to cut off the light of sensitizing wavelength range. Reference numeral 39 designates a photomask having an integrated circuit pattern, and reference numeral 21 denotes a photosensitive wafer. Reference numerals 41 and 42 designate mirrors for bending the optical path, reference numeral 44 denotes a concave mirror, and reference numeral 43 designates a convex mirror. These mirrors together constitute a one-to-one-magnification reflecting optical system. Designated by 46 is an alignment scope through which the photomask 39 and the wafer 21 are observed for alignment. The alignment scope 46 is inserted into the illuminating light path during alignment.
In the above-described apparatus, during the exposure, the photomask 39 and the wafer 21 are moved together in the direction of the arrows, and distortion is often created in the pattern image transferred to the wafer 21. This is, in some cases, due to the distortion created in the wafer itself when it was chemically treated in the preceding step, or, in other cases, to the distortion created during transfer.
The direction in which the transfer error occurs will be described with reference to FIG. 9 of the accompanying drawings.
In FIG. 9, reference numeral 21 designates a wafer and reference numeral 22 indicates a magnification error in the X direction. Reference numeral 23 indicates a magnification error in the Y direction, and reference numeral 24 indicates a right angle error. It is to be understood that the Y direction is the direction in which the photomask and the wafer 21 are moved and the X direction is a direction orthogonal thereto. Letter S designates an arcuate slit, namely, a mask illuminating area.
The transfer error in a projection exposure apparatus, in the X direction, is attributable to manufacturing error in the projection optical system, for example, the machining accuracy of the spherical mirror, while, in the Y direction, it is determined by the guide accuracy of a fluid bearing for moving the photomask and the wafer together relative to the imaging optical system. The right angle error is caused by the difference in degree of parallelism between the optical axis of the imaging optical system and the movement axis of the fluid bearing. As described above, the transfer error in the prior art is determined chiefly by the machining accuracy and the assembly accuracy. Therefore, the machining accuracy and assembly accuracy must be very high, and if a variation with time occurs the correction thereof is quite difficult.